It is a common practice within the semiconductor processing industry, to introduce a dopant impurity into one material by causing an impurity to migrate from another dissimilar material. A common example of causing an impurity to migrate from one material to another would be heat treating a semiconductor device to cause an impurity to be diffused from one material into a dissimilar, undoped material. An application of this procedure as it relates to a common semiconductor device, would include a dynamic random access memory (DRAM) device cell including a buried strap connecting a transistor to a trench capacitor. In a DRAM device including a buried strap, it is desirable to cause impurities to diffuse from a doped polysilicon electrode of the trench capacitor into the buried strap which connects the capacitor to the active area of the transistor, and then into the active area.
A common problem associated with causing an impurity to migrate from one material to another dissimilar material through an interface formed between two dissimilar materials, includes the effects of the interface upon the impurity migration. When an interface is formed between two materials, at least two known features can suppress the diffusion of a dopant impurity across the interface: the interface itself; and the grain structure of materials adjacent the interface.
This example, as it relates to the DRAM buried strap device, would involve the introduction of an impurity species from an amorphous/polycrystalline silicon buried strap, into the crystalline silicon structure comprising the active area of a transistor device formed within a semiconductor substrate. If the interfacial problem prevents extensive diffusion of dopant impurities from the buried strap and into the crystalline active area, an open will result and the electrical connection between the transistor and the capacitor will be faulty. Specifically, the device failure is caused when the dopant impurity in the trench is not diffused through the strap region, and into the active area resulting in electrical opens at low temperature conditions. Normally, the strap electrically connects a doped polycrystalline electrode of the trench capacitor to the active area through n-type doping.
Diffusion of an impurity through a material occurs fastest along grain boundaries. Thus, an electrically blocking crystal within the buried strap, with twinning and/or grain boundaries parallel to the interface between the active region, and generally orthogonal to the direction of the diffusion can disrupt the diffusion of impurities from the strap region into the active area. Generally, the buried strap is amorphous silicon, as deposited, and is converted to polycrystalline silicon which has a grain structure conducive to diffusion. Diffusion through a single crystalline silicon material, such as the active area, occurs at a much lower rate.
A common diffusion process used within the semiconductor industry, comprises utilizing a heating operation to simultaneously (a) convert an as-deposited amorphous silicon film into a polycrystalline structure, and (b) to cause diffusion of the dopant impurity from the amorphous/polycrystalline silicon region into the crystalline silicon region of the active area. The process is commonly used to urge diffusion of an n-type dopant impurity from the polycrystalline silicon of a buried strap into the crystalline silicon active area within a DRAM device. An associated problem with this process, however, is the formation of an electrically blocking, single crystalline grain structure forming in the polysilicon region near the interface between the polysilicon region and the crystalline silicon region. The single crystalline silicon structure of the active area "grows across" the interface and into the as-deposited amorphous silicon area if the interface between the two materials is smooth. In a trench DRAM, a heating process is used to produce buried strap polycrystalline silicon growth, and to enhance contact between the active area of the device and the trench capacitor by providing for the diffusion of a dopant impurity from one material (the polycrystalline buried strap being formed) to another material (the active area). It is found, however, that single crystal re-growth generates a dislocation in the active region which results in variable retention time failures.
One conventional approach to eliminating the formation of the electrically blocking, single crystalline region, is to prevent coherent nucleation at the interface between the two dissimilar materials. Coherent nucleation at the interface causes single grain growth adjacent the interface as a result of a smooth surface of the crystalline structure.
Another approach known in the art, which is directed to preventing the coherent nucleation of the crystalline surface, is using an "angle implant." The necessity for utilizing an angle implant is determined by the structure of the device. For example, a DRAM device using a buried strap structure connecting a trench capacitor to a transistor, requires angle implant because of the need to introduce a dopant into a surface which is substantially perpendicular with the substrate surface. This technique necessarily introduces an additional process step and thereby increases manufacturing complexity. Further, as devices become smaller, this step may become an even more significant source of other problems. This additional process step is more costly, requires more material and more time, and increases potential for error. Furthermore, the angle implant process is very difficult to control.
What is needed in the art is a method which helps generate a grain structure favorable for diffusion of dopant impurities in a region within a polycrystalline silicon material adjacent a crystalline silicon active area of a semiconductor device. Such a grain structure would enable, for example, diffusion of dopant impurities from a highly-doped polycrystalline silicon region of a trench capacitor, into a buried strap, then into the single crystalline active area of a DRAM device. By roughening the surface of a single crystalline material, such a grain structure is possible because a rough surface results in random nucleation sites at the interface which, in turn, will result in multi-grain growth in the polycrystalline silicon.
When a heating operation is used to cause the diffusion of an impurity from one material into another dissimilar material, crystal blocking at or near the interface significantly inhibits diffusion because impurity diffusion in perfect crystalline silicon is at least an order of magnitude slower than the diffusion along grain boundaries. In other words, "bulk diffusion" is at least an order of magnitude slower than the diffusion along grain boundaries. Depending on device dimensions, a single grain of silicon may be capable of slowing the intended dopant diffusion through, for example, a buried strap during thermal processing. When the strap is used to connect a trench capacitor to a DRAM transistor, the formed device structure would likely have low temperatures failures, as above.